Precise measurement and control of the positions of the materials during many polymeric processes are required to obtain consistent physical properties in the final product. Traditionally, only qualitative assessments of the stability of processes such as blown film, film casting and fiber spinning have been undertaken in the process industry. Therefore, the processes could not be optimized for maximum production rate with a minimum of physical property variation. Also, the effect of different additives and additive concentrations on stability can only be qualitatively determined with prior art systems. There is a need to be able to determine the stability of these processes while the materials are running and to be able to quantitatively determine the effect of additives and process variables on the stability of these processes. By using techniques that determine the relative position of the edges of the material and a means for relating the relative changes in those positions as a function either of time or frequency, one can determine the magnitude and type of instability. Since these processes operate at rates often in excess of 300 feet per minute, it is not desirable to try to measure stability using a contact system. Therefore, a non-contact system is more desirable.
Several investigations over the last few years have been undertaken to determine the stability of fiber spinning, film casting and blown film processing. All of these investigations used either very qualitative or off-line techniques as has been detailed by J. L. White and H. Tanaka, Journal of Applied Polymer Science, 26, 579 (1981) and S. Kase, Journal of Applied Polymer Science, 18, 3279 (1974), to name two. These investigations have used techniques that are not amenable to process optimization or process control on full scale equipment. Since much of the stability analyses were carried out in a qualitative manner, the determinations are observer dependent and as a result, are not consistent from day to day nor are they easily transportable from one process line to the next.
The stability of processes such as blown film, film casting and fiber spinning is what limits the rate and therefore the product profitability. In the industry today, almost all blown film, fiber spinning and film casting operations are operated at a limit well below the point where the material becomes unstable in order to maintain quality and reproducibility. Therefore, there is a need for a method and apparatus that provides accurate noncontact measurement of the stability of these types of processes such that the processes can be operated at maximum efficiency and such that resin development can be undertaken that provides materials which are optimized for maximum production rate with minimal stability problems.